IEEE 802 11 MAC Functionality Avaya Wireless implementation

« IEEE 802. 11 MAC Functionality

Avaya Wireless implementation of IEEE 802. 11 « Digital Signal Processor (Theseus) « IEEE 802. 11 MAC chip (Hermes) 4 -22 MHz HERMES Chip RADIO MODEM P HOST C Buffer & M I/F Fr agment D I S C C Management I HW A MDI MAC Control Functi on MMI THESEUS RADIO GPSIO Fl ash EPROM SRAM Boot. Flash 128 K*8 mi n 32 KB, max 2 MB Seri al EEProm

Avaya Wireless implementation of IEEE 802. 11 « Digital Signal Processor (Theseus) « IEEE 802. 11 MAC chip (Hermes) 4 -22 MHz HERMES Chip RADIO MODEM P HOST C Buffer & M I/F Fr agment D I S C C Management I HW A MDI MAC Control Functi on MMI THESEUS RADIO GPSIO Fl ash EPROM SRAM Boot. Flash 128 K*8 mi n 32 KB, max 2 MB Seri al EEProm

Avaya Wireless implementation of IEEE 802. 11 « Protocol functions programmed in FW, so flexible. « For use in station and access points (additional FW loaded when operating as access point) « Functions can be added over time, via upgrade utilities 4 -22 MHz HERMES Chip P HOST MAC C Buffer & M I/F Control Fragment D I S C C Function Management I HW A RADIO MODEM MDI MMI THESEUS RADIO GPSIO Flash EPROM Boot. Flash SRAM 128 K*8 min 32 KB, max 2 MB Serial EEProm

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Accessing the medium CSMA/CD station A defer station B CRS defer station C CRS collision « Adapters that can detect collisions (e. g. Ethernet adapters) « « Carrier Sensing: listen to the media to determine if it is free Initiate transmission as soon as carrier drops When collision is detected station defers When defer timer expires: repeat carrier sensing and start transmission

Accessing the medium CSMA/CA station B station C CRS defer CRS « Wireless LAN adapters cannot detect collisions: « Carrier Sensing - listen to the media to determine if it is free « Collision Avoidance - minimize chance for collision by starting (random) back -off timer, when medium is sensed free, and prior to transmission

CSMA/CA with MAC - level Acknowledgment Message ACK « Collisions still can occur (interference; incapability of sensing other carrier) « IEEE 802. 11 defines “low-level” ACK protocol « Provides faster error recovery « Makes presence of high level error recovery less critical

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

“Hidden stations” the problem A B C A sends to B C doesn’t detect that, so C might also start sending to B Collision of messages at B: both messages lost « Situation that occurs in larger cells (typical outdoor) « Loss of performance « Error recovery required

“Hidden stations” the solution A B C RTS: I want to send to B 500 bytes CTS: OK A, go ahead, so everybody quiet Data: the 500 bytes of data from A to B ACK: B received the data OK, so an ACK « IEEE 802. 11 defines: « « MAC level RTS/CTS protocol (Request to Send / Clear to Send) Can be switched off to reduce overhead (when no hidden nodes exist) More robustness, and increased reliability No interruptions when large files are transmitted

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Message fragmentation Hit A hit in a large frame requires re-transmission of a large frame Fragmenting reduces the frame size and the required time to re-transmit « IEEE 802. 11 defines: « MAC level function to transmit large messages as smaller frames (user definable) « Improves performance in RF polluted environments « Can be switched off to avoid the overhead in RF clean environments

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Multi-channel roaming « Avaya Wireless IEEE 802. 11 systems, support multi-channel roaming « Access points are set to a fixed frequency « Stations do not need to be configured for a fixed frequency « Stations switch frequency when roaming between access points « Stations “associate” dynamically to the access point with best signal, on power on « This implies « Easier configuration « Faster installation

Multi-channel roaming Channel 11 Channel 6 Channel 1

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Automatic rate select « Avaya Wireless PC Card, dynamically switches data-rate « Fall back to lower data-rate when communications quality decreases • out of range situations • Interference « Fall-back scheme: • 11 Mbps, 5. 5 Mbps, 2 Mbps, 1 Mbps « This implies « Operating at larger distances « Robustness in RF polluted areas

Automatic rate select « Avaya Wireless PC Card in AP-1000 and AP-500 is capable of supporting different data-rates “simultaneously”: « e. g. operates at “High” speed in communication to nearby station and at “Low” speed to station that is further away. « Data rate capability is maintained in “station association table” « Speed of IEEE Management - and Control frames use fixed speed determined as “IEEE Basic Rates”, and controlled by “Multi-cast Rate parameter”.

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Cell size / Multi Rate applications « Cell-size can be influenced by “Distance between APs” parameter: « Distance between APs = Large « Distance between APs = Medium « Distance between APs = Small -> -> -> large cell medium size cell small cell « Cell-size influences capacity per station in the cell « small cell physically accommodates smaller number of stations than large cell « bandwidth per station in small cell greater than in large cell « Cell size influences data-rate « larger distance between station and access-point may lead to lower data-rate

Cell size / Multi Rate applications « Mixture of cell-sizes accommodate mixed applications: « Office workers: • High physical station density • High bandwidth requirement • Small cell operating at high data rate • Distance between APs is small « Warehouse operations (such as forklift truck) • Low physical station density • Low bandwidth requirement (transaction processing) • Large cell operating at low data rate • Distance between APs is large

Multi Rate applications 2 Mbits/sec 1 Mbits/sec

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

In-cell Relay « IEEE 802. 11, in-cell relay: « Single radio module when used in the AP-1000 or AP-500 acts as repeater « Provides cells that are app. twice as large as with pre-IEEE wireless systems « Communication flows via access-point so overall transmission time increases relative to pre-IEEE 802. 11(or direct station to station communication) « This implies: « Larger cell size and consequently less need for access points and interconnecting infrastructure « Reduced performance in peer to peer communication within one cell compared to pre-IEEE 802. 11

In-cell Relay d In-cell relay: Larger cell (diameter = d >a) Lower throughput (data travels through air twice) a a No in-cell relay: Smaller cell (diameter = a<d ) Higher throughput (data travels through air once)

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Power Management « IEEE 802. 11, supports power management: « « nothing to send: station in sleep mode out-bound traffic stored in Access Point (out-bound = from AP to STA) station wake up only for Traffic Information Map (TIM) if messages: stay awake to receive them « This implies: « Prolonged battery life « Increase usability in hand-held equipment « Works best in application that have limited bandwidth requirements (transaction processing)

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Wired Equivalent Privacy « Optional security functionality (factory “installed”) « Encryption based on RC 4 (1988 RSA algorithm) « Stream cipher 64 or 128 bits key « Used by Netscape, Microsoft, Oracle and Lotus (80 million users) « Used for data encryption « Used for shared key station authentication

IEEE 802. 11 features « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)

Wireless Distribution System « IEEE 802. 11, WDS means « Multiple wireless “ports” inside the access-point, to wirelessly interconnect cells (access-points connecting to other access-points) « pre-IEEE 802. 11, did not support WDS: « Three ports exist in one access-point (one Ethernet, and two wireless cells) « One wireless backbone extension can be made (using two radio modules in the access-point) « WDS allows: « Extending the existing infrastructure with wireless backbone links « Totally wireless system without any wired backbones, needed in locations where large areas are to be covered and wiring is not possible

Wireless Distribution System Channel 11 Channel 6

IEEE 802. 11 features Module summary « ACK protocol « Medium reservation (RTS/CTS) « Fragmentation « Multi-channel roaming « Automatic data-rate fall-back « Cell size / Multi-rate applications « In-cell relay « Power Management « Wired Equivalent Privacy (WEP) « Wireless Distribution System (WDS)